1. Field of the Invention
The Present invention relates to a magneto-optical recording medium, such as a magneto-optical disk, magneto-optical tape, magneto-optical card, or the like, for use with a magneto-optical recording and reproduction apparatus, and more particularly to a magneto-optical recording medium capable of magnetically-induced super-resolution readout, and a method of reading information recorded on such a magneto-optical recording medium.
2. Description of Related Art
In recent years, magneto-optical disks have been attracting much attention as external storage media for computers. The magneto-optical disk uses an external magnetic field and a laser beam to form recorded bits of submicron size on the medium, and can achieve a drastic increase in storage capacity as compared with the more conventional external storage media such as floppy disks and hard-disks.
A currently commercialized 3.5-inch magneto-optical disk has tracks arranged at 1.4-.mu.m pitch over a concentric area 24 to 40 mm wide across the disk radius, and contains recorded marks of minimum length of 0.65 .mu.m recorded along the circular direction thereof, achieving an approximately 230-megabyte storage capacity per side. Since one 3.5-inch floppy disk can hold about 1.3 megabytes of data, it follows that the 3.5-inch magneto-optical disk has a storage capacity 200 times that of the 3.5-inch floppy disk.
When recording an information signal on a rewritable magneto-optical disk, a light beam is applied to heat a small area in a magneto-optical recording layer, and the magnetization in the heated area is reoriented (to form a recorded bit) by simultaneously applying an external magnetic field according to the information to be recorded. Readout of recorded information is done by using the Kerr effect, according to which, when a light beam is applied to the magneto-optical recording layer, the plane of polarization of the reflected light is rotated according to the direction of the magnetization.
Such magneto-optical disks have come to be placed among the most promising storage media for storing ever increasing amounts of data. Because of rapid advances in multimedia applications, the need for higher storage capacities has been increasing. To increase the storage capacity of a magneto-optical disk, that is, to increase the recording density thereof, it is imperative that the bit length be reduced and the spacing between bits be made smaller.
In conventional magneto-optical recording and readout, however, the recording and readout performance is limited by the size of the light beam focused on the medium (i.e., the size of the beam spot). To read bits recorded at shorter frequencies than the beam diameter, the beam should be focused into a smaller spot, but since the size of the beam spot is determined by the wavelength of the light source and the numeric aperture of the objective lens, there is a limit on how small the beam spot can be made.
To achieve higher-density recording, there has been proposed a magnetically-induced super-resolution (MSR) medium that permits readout of recorded bits of size smaller than the beam spot, along with a recording and reading method using the MSR medium (Japanese Patent Application Laid-Open Nos. 1-143041 (1989), 3-93058 (1991), 4-271039 (1992), 5-12731 (1993), etc.). This recording and reading method uses a recording medium consisting of a plurality of magnetic layers, formed one on top of another, whose magnetic properties vary with temperature, and by utilizing the temperature distribution in the recording medium formed within the beam spot, produces an effect equivalent to the effect that would be obtained when the beam was focused into a smaller spot. This ensures reliable readout of recorded information even if the recorded bits are smaller than the size determined by the beam spot. The MSR medium and the recording and reading method for the same will be described below.
First, a description will be given of the method disclosed in Japanese Patent Application Laid-Open No. 1-143041 (1989) (hereinafter referred to as the first prior art method) which a is generally known as the front aperture detection (FAD) method in which a recorded mark is read out from a low-temperature region with a high-temperature region within the beam spot acting as a mask region As shown in FIG. 1, the recording medium has a structure in which a readout layer 61, a switch layer 62, and a recording layer 63 are formed one on top of another in this order on a substrate. At room temperature, the magnetization direction of the readout layer 61 is the same as that of the recording layer 63 because of the exchange coupled force acting via the switch layer 62. However, when the temperature rises by applying a reading laser light, the exchange coupled force from the recording layer 63 is lost in an area where the Curie temperature of the switch layer 62 is exceeded (the high-temperature region); therefore, the magnetization direction of the readout layer 61 in that area is aligned with that of an externally applied read magnetic field (Hr). As a result, the high-temperature region acts as a mask to cover an underlying recorded bit 65 formed on a recording track 64, so that only a recorded bit 65 transferred from the recording layer 63 to the readout layer 61 in the low-temperature region is read out.
The method disclosed in Japanese Patent Application Laid-Open No. 3-93058 (1991) (hereinafter referred to as the second prior art method) is generally known as the rear aperture detection (RAD) method in which a recorded bit is read out from a high-temperature region with a low-temperature region within the beam spot acting as a mask region. As shown in FIG. 2, the recording medium has a structure in which a readout layer 71 and a recording layer 72 are formed one on top of the other in this order on a substrate. Immediately before applying a reading laser light, an initializing magnetic field is applied by an initializing magnet 73 so that the magnetization direction of the readout layer 71 alone is aligned with that of the initializing field as each recorded bit 75 formed on a recording track 74 passes through the initializing field. At this time, the recording layer 72 retains the state of each recorded bit 75. Immediately after the application of the initializing magnetic field, the readout layer 71 is acting as a mask covering data recorded in the recording layer 72. Then, when the reading laser light is applied, the temperature of the readout layer 71 acting as the mask rises. When the exchange coupled force with the recording layer 72 becomes greater than the coercive force of the readout layer 71, the direction of magnetization in the recording layer 72 is transferred. That is, the readout layer 71 is unmasked in this high-temperature region, from which a recorded bit 75 is read out.
The method disclosed in Japanese Patent Application Laid-Open No. 4-271039 (1992) (hereinafter referred to as the third prior art method) is generally known as the RAD double-mask method in which a recorded mark is read out from an intermediate-temperature region with high- and low-temperature regions within the beam spot both acting as mask regions. As shown in FIG. 3, the recording medium has a structure in which a readout layer 81, a control layer 82, an intermediate layer 83, and a recording layer 84 are formed one on top of another in this order on a substrate. As in the case of the second prior art method, immediately before the irradiation of the reading laser light, an initializing magnetic field is applied by an initializing magnet 85, to align the magnetization direction only of the reading layer 81 and control layer 82 with that of the initializing magnetic field. At this time, the recording layer 84 retains the state of each recorded bit 87. In a region (low-temperature region) just subjected to the initializing magnetic field, the readout layer 81 is acting as a mask covering recorded bits recorded in the recording layer 84. When the reading laser light is applied, the exchange coupled force from the recording layer 84 is lost in an area (high-temperature region) where the temperature rises above the Curie temperature of the control layer 82, so that the magnetization direction of the readout layer 81 in that area is aligned with that of an externally applied read magnetic field (Hr). As a result, the high-temperature region acts as a mask covering recorded bits 87 formed on a recording track 86. In this way, with the low- and high-temperature regions both acting as masks, the intermediate-temperature region sandwiched between them forms a transfer region (an aperture) from which a recorded bit is read out.
The method disclosed in Japanese Patent Application Laid-Open No. 5-12731 (1993) (hereinafter referred to as the fourth prior art method) is generally known as the central aperture detection (CAD) method. As shown in FIG. 4, the recording medium has a structure in which a readout layer 91 and a recording layer 92 are formed one on top of the other in this order on a substrate. The readout layer 91 is formed from a magnetic film whose easy axis of magnetization is oriented in an in-plane direction at room temperature but becomes oriented in a direction perpendicular thereto at high temperatures, so that the high-temperature region where the temperature has risen by applying the reading laser light is coupled with the recording layer 92 by the exchange coupled force. As a result, the low-temperature region of the readout layer 91, where the magnetization is oriented in the in-plane direction, acts as a mask to cover recorded bits formed on a recording track 93, while in the high-temperature region, a recorded bit is transferred to the readout layer 91 because of the exchange coupled force with the recording layer 92, and the recorded bit is thus read out from the transfer region.
In this way, each of the prior art methods is able to read out a recorded bit from a region smaller than the spot diameter of the reading laser light, thus in effect achieving a resolution equivalent to the resolution that would be obtained when the readout was done using a beam spot smaller than the spot diameter of the reading laser light.
However, the above-described prior art methods have the following shortcomings. The first prior art method can reduce the overall size of the apparatus since it does not require the provision of an initializing magnet, but is not effective in suppressing crosstalk in which data from adjacent recording tracks are mistakenly read out, since it reads data from a low-temperature region that forms a wide transfer region. On the other hand, the second prior art method is effective in suppressing crosstalk since it reads data from a high-temperature region, but is not effective in reducing the size of the apparatus since it requires the use of an initializing magnet. The third prior art method is effective in suppressing crosstalk and permits an increase in read output, but is not effective in reducing the size of the apparatus, since it requires the use of an initializing magnet, as in the case of the second prior art method: The fourth prior art method does not require the use of an initializing magnet, but cannot obtain a high read output since a transition region, where the magnetization of the readout layer makes a transition from the in-plane to the perpendicular direction, is wide.